Sometimes, you'll find yourself thinking, "Why is xx diphone not in my voicebank?"
There's a few ways that someone might respond to this.
A. Ignore it, and figure out a workaround without adding samples
B. Try to add random samples, as if this was your basic CV Japanese with extras
C. Look at the existing structure and see how to fit your idea in

When Mr. Hua said to distribute Arpasing USTs as plain, unconverted USTs, his reasoning was that each voicebank would be different and that it's...

uh, yes and no. Arpabet. and therefore arpasing, is designed specifically for English and the sounds in English, so it'd be difficult to represent another language with the same system. Like, there's nothing stopping you from trying, but its a bit like trying to force a square peg through a round hole.

The arpasing assistant plugin can only convert English words. Using this guide to write an arpasing reclist will simply result in something you can use with Moresampler's arpasing OTO generator.

If you write a list in a similar format (with an index.csv, etc) but with your own custom phonemes, you can use this tool to generate the OTO instead. https://github.com/adlez27/oto-generator However, it will not analyze the audio and adjust the timing the way Moresampler does.